The present invention relates to a toroidal-type continuously variable transmission and, for example, to a toroidal-type continuously variable transmission for use in a vehicle such as a car.
Conventionally, as a toroidal-type continuously variable transmission, for example, there is known a transmission which has such a structure as shown in FIG. 1.
Specifically, in the interior of a housing (not shown), an input disk 1 and an output disk 2 are disposed in such a manner that they are concentric with each other but are opposed to each other. An input shaft 3 is penetrated through the axial core portion of a toroidal transmission part in which the input disk 1 and output disk 2 are disposed. On one end of the input shaft 3, there is disposed a loading cam 4. This loading cam 4 is structured such that it can transmit the power (rotational force) of the input shaft 3 to the input disk 1 through a cam roller 5.
Also, between the input disk 1 and output disk 2, there are interposed a plurality of trunnions swingable about their respective support shafts (not shown) disposed at positions twisted with respect to the center axes of the two disks 1 and 2 (in other words, between the input disk 1 and output disk 2, there are interposed a plurality of trunnions swingable about their respective support shafts (not shown) each of which is disposed at a positions that is perpendicular to an axial line of the input shaft 3 and does not intersect the axial line thereof (this physical relationship is referred as xe2x80x9ca twisted positionxe2x80x9d.).
The input disk 1 and output disk 2 are substantially the same in shape and are also arranged symmetric, while their respective opposed surfaces are respectively formed as toroidal surfaces which, when they are viewed in the axial-direction section thereof, cooperate together in forming a substantially semicircular shape. Within a toroidal cavity defined by the toroidal surfaces of the input disk 1 and output disk 2, a pair of power roller bearings 6 and 7 are disposed for power transmission in such a manner that they are in contact with the input disk 1 and output disk 2.
By the way, the power roller bearing 6 comprises a power roller 6a (which corresponds to an inner ring forming the power roller bearing 6) capable of rolling on the toroidal surfaces of the input disk 1 and output disk 2, an outer ring 6b, and a plurality of rolling elements (steel balls) 6c. On the other hand, the other power roller bearing 7 comprises a power roller 7a (which corresponds to an inner ring forming the power roller bearing 7) capable of rolling on the toroidal surfaces of the input disk 1 and output disk 2, an outer ring 7b, and a plurality of rolling elements (steel balls) 7c. 
That is, the power roller 6a serves also as the inner ring which is a component of the power roller bearing 6, and the power roller 7a serves also as the inner ring which is a component of the power roller bearing 7. In this structure, the power roller 6a is mounted pivotally and rotatably on a trunnion 10 through a pivot shaft 8, the outer ring 6b and the plurality of rolling elements 6c, and also is supported so as to be inclinable about a shift shaft 0 which provides the center of the toroidal surfaces of the input disk 1 and output disk 2.
On the other hand, the power roller 7a is mounted pivotally and rotatably on a trunnion 11 through a pivot shaft 9, the outer ring 7b and the plurality of rolling elements 7c, and also is supported so as to be inclinable about a shift shaft 0 which provides the center of the toroidal surfaces of the input disk 1 and output disk 2. And, lubricating oil (traction oil) having large viscous frictional resistance is supplied to the contact surfaces of the input disk 1 and output disk 2 with respect to the power roller 6a and power roller 7a, in such a manner that the power input to the input disk 1 can be transmitted to the output disk 2 through the film of the lubricating oil, power roller 6a and power roller 7a. 
By the way, the input disk 1 and output disk 2 are connected with the input shaft 3 through needle roller bearings 12 such that they are independent of the input shaft 3 (that is, they are held in a state where they are not influenced directly by the power of the input shaft 3). On the output disk 2, there is disposed an output shaft 14 which is arranged in parallel to the input shaft 3 and also supported rotatably on the housing (not shown) through angular contact ball bearings 13.
In the toroidal-type continuously variable transmission 20, the power of the input shaft 3 is transmitted to the loading cam 4. In case where the loading cam 4 is rotated due to this power transmission, power generated by the rotation of the loading cam 4 is transmitted through the cam roller 5 to the input disk 1, so that the input disk 1 can be rotated. Further, power generated by the rotation of the input disk 1 is transmitted through the power rollers 6a and 7a to the output disk 2. As a result of this, the output disk 2 is rotated integrally with the output shaft 14.
In speed change, the trunnions 10 and 11 are moved by a slight distance in the directions of the shift shafts 0, respectively. That is, due to the movements of the trunnions 10 and 11 in the axial direction thereof, the intersection between the rotation axes of the power rollers 6a, 7a and the axes of the input and output disks 1, 2 is shifted slightly. This shift loses balance between the rotation peripheral speeds of the power rollers 6a, 7a and the rotation peripheral speed of the input disk 1. In addition, due to the component of the rotation drive force of the input disk 1, the power rollers 6a, 7a are allowed to inclinedly roll around the shift shafts 0, whereby the power rollers 6a, 7a are allowed to incliningly roll on the curved surfaces of the input and output disks 1, 2. As a result of this, the speed ratio is changed, so that the speed of the vehicle is decreased or increased. As a toroidal-type continuously variable transmission having the above structure, for example, there is known a conventional transmission which is disclosed in JP-B-2-49411U.
In the above-mentioned toroidal-type continuously variable transmission, when driven, there is a fear that the power roller retainers of the power roller bearings interposed between the input and output disks can be damaged or broken. The reason for this is as follows: that is, when this toroidal-type continuously variable transmission is driven with a high load and high torque, the inner ring raceways of the power roller bearings, which have received loads from the input and output disks, are respectively deformed into an elliptic shape; and, when the rolling elements held by the retainers roll along the thus-deformed raceways, the rolling elements apply loads to the retainers. As a measure to deal with problem, there is known a method of enhancing the strength of the retainers. However, in this method, in case where the strength of the retainers is increased, seizure can occur between the contact portions between the retainers and power roller inner rings or outer rings, or the power roller inner rings or outer rings can be broken by the retainers.
The present invention aims at eliminating the above problems found in the conventional toroidal-type continuously variable transmission. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission in which the retainers of the power roller bearings are made of high-strength brass to thereby be able to optimize the strength of the retainers.
In attaining the above object, according to the invention, there is provided a toroidal-type continuously variable transmission, comprising: an input shaft; an input disk rotatable together with the input shaft; an output disk disposed concentrically with the input disk and supported rotatable with respect to the input disk; a plurality of trunnions respectively swingable about support shafts disposed at twisted positions with respect to the center axes of the input and output disks; a plurality of shift shafts respectively supported on their associated trunnions; a plurality of power rollers respectively rotatably supported on the shift shafts and interposed between the input and output disks; and thrust ball bearings for supporting thrust loads applied to the power rollers, wherein each of the thrust ball bearing comprises: an inner ring raceway formed in the outer peripheral surface of the power roller; an outer ring including an outer ring raceway formed in the inner peripheral surface thereof; a plurality of balls interposed between and rollable along the inner ring raceway and outer ring raceway; and, a retainer for holding the balls, wherein the retainer is formed of high strength brass and the high strength brass has a micro structure in which the xcex1 phase thereof is 40% or more in the area ratio thereof.
Preferably, the hardness of the retainer may be set at HB80 or higher.
Further, preferably, the xcex1 phase is set in the range of 40% to 80%.